Here we go!
"Our new Dual Beam sonar units are optimized to show the greatest bottom definition using a narrow (20°) beam yet can still indicate fish found in the wide (45°) beam when manually switched. Dual Beam is ideal for a wide range of conditions - from shallow to very deep water in both fresh and salt water. Please note that boat speed, wave action, bottom hardness, water conditions and transducer installation can all affect depth capability. "
I found the one reference above that may clear up the extremely fuzzy Garmin speak about fresh/salt when improperly used to mean deep and not as deep as applied to dual beams and dual freq beams..
Note that in this answer to a FAQ, the Garmin folks say that the dual beam is fresh and salt water capable...
Certified Stan Gibbs Cape Cod Canal Classic Flyrodder